The present invention relates to a diphosphine, a process for preparing it and to the use of the diphosphine for stabilizing a thermoplastic polymer against thermal discoloration. Furthermore, the present invention relates to a polymer composition containing a thermoplastic polymer and the diphosphine.
It is generally known in the art that most of the known thermoplastic polymers are affected to a certain extent when they are exposed to heat. The deterioration of the polymers often results in yellowing of the polymer and in degradation of its molar mass. Therefore, it is common to treat thermoplastic polymers with stabilizers. A large variety of stabilizers have been suggested in the prior art for the various thermoplastic polymers.
It has been suggested to utilize certain organic phosphites, phosphonites or phosphines, optionally in combination with epoxides or polydialkylsiloxanes, for stabilizing polycarbonates against thermal or thermal-oxidative degradation.
For example, U.S. Pat. No. 4,092,288 discloses color stable polycarbonate compositions consisting of an aromatic polycarbonate resin having in admixture therewith 0.005 to 0.5 weight percent of a triaryl, trialkyl, tri(alkylaryl) or alkyl-aryl phosphine, preferably triphenylphosphine.
U.S. Pat. No. 4,627,949 discloses a process for preparing shaped articles from polycarbonates stabilized with phosphanes by a devolatilizing injection-moulding or extrusion process. The phosphanes have the formula (R).sub.2 PR.sup.1 wherein R is an unsubstituted or substituted C.sub.6 -C.sub.14 -aryl radical and R.sup.1 =R or an unsubstituted or substituted C.sub.1 -C.sub.18 -alkyl radical.
U.S. Pat. No. 4,835,202 discloses the use of (hydroxyphenyl)phosphine derivatives for stabilizing a wide variety of polymers against oxidative, thermal and actinic degradation.
Unfortunately, the suggested stabilizers are volatile at the high temperatures which are applied for extruding polycarbonates.
U.S. Pat. No. 4,145,525 discloses polyalkylene carbonates of improved thermal stability. At least a portion of the free hydroxyl groups is reacted with a hydroxyl reactive phosphorus compound whereby the active hydrogen on the hydroxyl group is replaced by an oxygen-phosphorus bond. Unfortunately, the oxygen-phosphorus bond can be easily hydrolyzed by acids. Accordingly, the end-capped groups are relatively instable in the presence of acids. However, traces of acids are often present when polycarbonates are washed and later extruded.
U.S. Pat. No. 4,474,937 discloses phosphorus-modified polyester carbonate resins. The repeating unit within the polymer can be a phosphonite, phosphite, phosphonate or phosphate species. The amount of phosphorus present in the polymer is 1-1000 ppm, preferably 1-100 ppm, based on the weight of the polymer. The resins exhibit improved thermal-oxidative stability compared to non-modified polyester carbonates.
U.S. Pat. No. 4,444,978 discloses the preparation of carbonate polymers having increased thermal stability by incorporating into the polymer chain an oligomer which has the formula: EQU H--[O--R--O--P(OR.sub.1)].sub.n --O--R--OH.
R is the divalent residue of a dihydric mononuclear or a dihydric polynuclear phenol, R1 is an alkyl, aralkyl or alkaryl group having 1 to 25 carbons, and n is a number having an average value of 1 to 200. From 10 to 2000 ppm, preferably from 100 to 1000 ppm of the oligomer are copolymerized with the copolymer.
Although a wide variety of stabilizers exist for the various types of thermoplastic polymers, it is still desirable to provide new stabilizers in order to meet the ever increasing quality requirements for thermoplastic polymers.
One object of the present invention is to provide new compounds which can be used for stabilizing thermoplastic polymers, such as polycarbonates, against thermal discoloration. It is a preferred object of the present invention to provide a new stabilizer for thermoplastic polymers which is less volatile at high temperatures than the phosphines and phosphanes disclosed in U.S. Pat. Nos. 4,092,288 and 4,627,949.